It has been long established that the expression of a memory does not necessarily reflect the underlying strength of the memory trace. In particular, it is known that a conditioned response based on an associative memory might reflect not only the associative "strength" of the memory, but also such factors as the animal's motivation to respond, competition between other cues present at the time of training or testing, and whether the memory was stored in a form which is readily retrievable. In the present series of experiments, we will examine several such determinants of responding in a simple organism using an associative learning task. For nearly thirty years, a "simple systems" approach has dominated the exploration of the neurophysiological mechanisms underlying memory formation. For instance, using model systems such as habituation and sensitization in Aplysia and associative eyeblink conditioning in the rabbit, many of the biophysical and biochemical events which are critical to memory formation have been elucidated, and in the case of vertebrate models, an appreciation of the anatomical substrates of certain memories has been achieved. While this approach has been quite successful, much of what psychologists consider to be fundamental memory processes have received relatively little attention. In the present proposal, several general classes of experiments are planned, focusing on the Interaction between training variables (i.e., number and distribution of training trials) and the induction and subsequent retentIon of an associative memory in Hermissenda. Many of these variables have been assessed at the behavioral level in vertebrate species. For instance, it is well established that retention is influenced by the number of initial training trials, and the rate at which the successive trials are presented. Moreover, it has been demonstrated that postconditioning exposure to components of the initial learning event (e.g., the CS, the US, or the training context) can either facilitate or retard subsequent responding depending on such factors as the number of exposures to these stimuli, and the strength of the original memory trace. Using Hermissenda we will attempt to establish parameters by which facilitated reacquisition following memory degradation may be observed, and to determined when post- training cuing treatments (i.e., brief exposure to the CS, the US, or the training context) are effective. These experiments, which are conceptually based in the vertebrate learning literature, are to be undertaken in Hermissenda for several reasons. First, given the capacity to condition Hermissenda using in vitro techniques, it should be possible to examine biophysical and biochemical properties of cells involved in memory formation during each stage of memory processing. Second, associative learning in Hermissenda occurs within a narrow range of parameters, and depending on the number of training trials, the effective retention interval can be as short as several minutes or as long as seven to ten days. In total, these two features (capacity for neurophysiological analysis during in vitro procedures and strict control of the capacity for retention) provides a unique opportunity to study the neurophysiological mechanisms underlying these simple psychological processes.